The words on every report, explained.
Nothing on an AcoustaPLOT report should require looking anything up separately — but if something does, this is the page it sends you to.
The noise figures
Lden
“Day-evening-night level” — a single figure summarising noise exposure across a full 24 hours, but not a simple average. Evening noise (19:00–23:00) is weighted 5 dB higher, and night noise (23:00–07:00) 10 dB higher, than the same level during the day — because the same traffic is more disruptive when people expect quiet. It's the standard descriptor behind the Defra Strategic Noise Maps and the WHO guideline this site checks a property against.
Lnight
The noise level for the night period alone (23:00–07:00), with no day or evening data folded in. Reported alongside Lden because a location can have an unremarkable Lden and still have a genuinely disruptive night-time level, or the other way round — sleep disturbance is judged against Lnight specifically, not the blended Lden figure.
dB(A)
Decibels, A-weighted — a scale adjusted to roughly match how sensitive human hearing is at different frequencies, so a reading in dB(A) tracks perceived loudness more closely than a raw, unweighted decibel figure would. Virtually every environmental noise limit and guideline, including every figure on an AcoustaPLOT report, is expressed this way.
Logarithmic (energy) addition
Decibels don't add arithmetically — two sources each at 60 dB combine to about 63 dB, not 120 dB, because the decibel scale is logarithmic. Where a property's noise comes from more than one mapped source, the risk-band figure combines them this way rather than simply picking the louder one or summing the numbers directly.
The risk band, and what sits behind it
Risk band
AcoustaPLOT's own seven-tier rating (LOW to SEVERE), built to sit either side of the SOAEL and UAEL proxies below rather than an arbitrary scale. The full table, with the Lden/Lnight range for each band, is on the Methodology page.
LOAEL
Lowest Observed Adverse Effect Level — the point from which noise starts to have any identifiable effect on health or quality of life at all, per the Noise Policy Statement for England (NPSE). Below it, effects may still be present but aren't judged significant.
SOAEL
Significant Observed Adverse Effect Level — the NPSE threshold above which adverse effects on health and quality of life become significant, not just noticeable. AcoustaPLOT uses a 63 dB proxy for this, stated on the Methodology page.
UAEL
Unacceptable Adverse Effect Level — AcoustaPLOT's own further category above SOAEL, for noise exposure severe enough that no amount of mitigation should be considered to make it acceptable. NPSE itself only defines NOEL, LOAEL and SOAEL, not a fourth UAEL level — this one is AcoustaPLOT's own extension of that framework, not a government-defined threshold. AcoustaPLOT uses a 69 dB proxy for it, stated on the Methodology page.
Uncertainty range
Every AcoustaPLOT figure is a modelled estimate, not a measurement, and states the range it could realistically fall within — ±3–5 dB where it's read from Defra's own mapping, ±5–8 dB where it's calculated from traffic data instead. Because risk bands are 5 dB wide, a result near a boundary can genuinely sit in the band either side of it.
A risk band tells you where a property sits. It doesn't tell you what to do about it — that's what a Barrier Design or Complete report is for, and the Standards page lays out exactly which standard each of those recommendations is checked against.
The method, in short
AADF
Annual Average Daily Flow — the Department for Transport's count of vehicles using a road, averaged across a full year to smooth out seasonal and day-of-week swings. It's the traffic input to the CRTN calculation wherever a property falls outside Defra's own mapped coverage.
CRTN
Calculation of Road Traffic Noise (1988) — the UK method for turning a traffic count into a noise level. Used as the fallback wherever a property sits outside Defra's Strategic Noise Map coverage. Full detail on the Standards page.
ISO 9613-2
The international standard for how sound attenuates as it travels outdoors — divergence, atmospheric absorption, ground effect and diffraction around a barrier. It's the method behind every Acoustic Barrier Design report's recommended fence height and length. Full detail on the Standards page.
Insertion loss
The reduction in noise level a barrier actually achieves, in dB — the figure a fence height is chosen to deliver. It depends on the barrier's height and length, how close it sits to the source or the receiver, and the frequency content of the noise; a taller fence closer to the noise source generally outperforms the same height set further back.
Defra END
The Environmental Noise Directive strategic noise maps Defra produces and updates periodically (Round 4, 2022, is the current basis for this site) — modelled road and rail noise bands for most of England, read directly wherever a property falls within their coverage. Full detail on the Standards page.
Missing something you had to look up elsewhere? Tell us at noise@acoustaplot.com — this page grows every time someone does.
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